A J Munro

Abstract

This report gives a brief synopsis of the recent flood event on the Lower
Waikato River, located approximately 50 km south of Auckland, New Zealand. Extremely
high river levels were experienced on the Waikato and Waipa Rivers between
9-20 July due to widespread, and in places record rainfall. As a result,
urban centres including Hamilton, Ngaruawahia, Huntly, and Mercer were
inundated. Farms adjacent to the Waipa River, Whangamarino Wetland, and
properties fringing Lakes' Taupo and Waikare were also affected. This
event came off the back of a previous deluge that occurred only a week
before. These back to back deluges (along with already saturated ground
conditions) and successive frontal bands meant that rivers were unable to
cope with the copious amounts of water entering the catchment. Flood
warnings and management systems generally worked very well, and the Lower
Waikato Waipa Control Scheme performed as per design.

The Waikato Regional Flood
Event of 9-20 July 1998

Background

The Waikato River is New Zealand's longest river (425 km), forming the centrepiece of
the Waikato Region. It begins its long journey on the northeastern slopes of Mt
Ruapehu (2797 metres), flowing into Lake Taupo. From the lake, the river cuts its way
through the volcanic deposits of the central plateau, across which are eight hydro-electric dams, and onto the flat often swampy lowlands between Cambridge and Mercer.
The river finally discharges into the Tasman Sea at Port Waikato, approximately 30 km
south of Auckland. Overall, it drains water from 18,650 km2 or 12% of the area of the
North Island.

Introduction

Very high river levels were experienced over the whole region during 9-20 July due to
widespread, and in places record, rainfall. This report gives a brief overview of the
flood event from the main catchments affected. Due to the size and nature of this
event, a direct comparison is made where possible with the February 1958 Flood Event
using both historical rainfall and river flow data (Tables 1 & 2). Information contained
in this report is provisional, as further analysis and investigations are being undertaken.

Rainfall

Heavy rainfall was experienced throughout the region between 8-16 July 1998 with the
heaviest falls being recorded in the Taupo, Rangitoto, Coromandel/Kaimai, Hapuakohe
and Hunua Ranges. This was the second deluge, the other occurring less than a week
earlier. These back to back deluges saturated the ground to the extent that river levels were unable to return to normal between the events. The upper catchment rivers therefore reacted very quickly upon the arrival of this second deluge.

The design rainfall frequency for the Lower Waikato Waipa Control Scheme is 200-250mm over three days.

The 7-day total rainfall from 8-15 July for the relevant rainfall recorders are presented
in Table 1. Most of the rain fell 9-11 July, with Ngaroma (near Te Kuiti) receiving its
July normal in less than two days. The 7-day figures are compared with rainfall in a
'normal' July.

Table 1 : Rainfall figures

Rainfall Site

Catchment

8-15 July 1998, mm

Normal July, mm

% Above Normal

Feb 1958, mm

Waikato/Waipa Catchments

Pukekawa

Lower Waikato

165

150

10

-

Mangatangi

Mangatangi

316

207

53

-

Maungakawa

Mangawara/Piako

202

150

35

325?

Hamilton

Waikato

191

123

55

324

Otewa

Waipa

196

155

26

-

Ngaroma

Waipa

361

223

62

340

Te Kuiti

Mangaokewa

260

170

53

-

Hauraki/Coromandel Catchments

Smiths

Kauaeranga

165

130

27

-

Pinnacles

Kauaeranga

460

435

6

-

Golden Cross

Ohinemuri

273

296

-8

-

Maukoro

Piako

172

120

43

-

Waharoa

Waitoa

138

128

8

-

Wharekawa

Waihou

275

250

10

-

Te Aroha

Waihou

150

166

-11

-

Kaimai

Waihou

370

175

111

-

Taupo/Awakino Catchments

Wharekiri

Mangaokewa/Waipa

250

197

27

-

Waitanguru

Awakino/Mokau

286

304

-6

-

Turangi

Tongariro

195

150

30

507

Figure 1 : Map of the Waikato Region, showing the major rivers affected in this flood event.

River Flows

Recorded peak flows are presented in Table 2 below.

Table 1 : River flows (cumecs)

Location

Peak Flow July 1998 and typical values (cumecs)

Peak Date July 1998

Comparison cumecs

Taupo/Awakino Rivers

Poutu Intake (Tongariro)

660

-

9th

572 on 2/7/98

Tauranga-Taupo

218

13

10th

204 on 2/7/98

Awakino @ SH3

282

-

9th

-

Waipa/Waikato Rivers

Mangaokewa @ Te Kuiti

122

11

9th

87 on 2/7/98

Waipa @ Otewa

recorder damaged durning flood

Waipa @ Otorohanga

418

52

10th

625 in 1958

Waipa @ Whatawhata

776

165

13th

1130 in 1958

Waikato @ Hamilton

807

306

15th

905 in 1958

Waikato @ Ngaruawahia

1491

456

12th

1482 in 1958

Waikato @ Huntly

1490

490

15th

1540 in 1958

Waikato @ Rangiriri

1490

507

15th

-

Waikato @ Mercer

1575

545

16th

1260 in 1958

Hauraki Basin/Coromandel Rivers

Waihou @ Okauia

180

33

11th

-

Waihou @ Te Aroha

190

53

16th

-

Waihou @ Puke Bridge

(tidal)

-

15th

-

Waitoa @ Waharoa

25

3

12th

-

Waihou @ Mellon Rd

68

11

12th

-

Piako @ Kiwitahi

72

4

11th

-

Piako @ P-T Road

168

17

12th

-

Piako @ Maukoro Landing

(tidal)

-

13th

-

Kauaeranga @ Smiths

353

11

26th

-

Ohinemuri @ Karangahake

295

21

15th

-

Lake Taupo Levels and Gate Settings

Directly prior to the second event (9 July), Lake Taupo was at a level of 356.94 metres,
still 0.31 metres below the maximum winter control level. The control gates were set to
minimum through the initial stages to minimise flooding downstream of Karapiro. The
initial push of water through the hydro dams was to get rid of as much water as possible through the already swollen Hydro lakes before the Waipa peak arrived at Ngaruawahia.

The Taupo control gates were again set to minimum on 15 July to reduce lower Waikato
River levels. Between 16-17 July, river levels generally declined, with Lake Taupo
swelling to its highest level in 40 years (357.49 m) due to continued high tributary
input. In fact, it has been calculated that the total inflow for Lake
Taupo for July 98 is the biggest monthly inflow since records began in 1905.

The Tongariro Power Development scheme (diversion of foreign waters) was shut down
when Lake Taupo reached its winter maximum control level of 357.25 metres.

On 17 July, Lake Taupo control gates and Karapiro Dam discharges were increased to
reduce Lake Taupo levels. The control gates were maintained at their maximum setting
until the level of lake fell significantly below the winter maximum control level of
357.25 m (this was reached on Monday 3 August, more than two weeks after the gates
were fully opened).

The main effects from these high lake levels were:

foreshore flooding and erosion at lakeside settlements due to wave/wind action. This was particularly evident on the foreshore adjacent to Taupo township;

increased pressure on sewerage and stormwater systems in some areas (the main event occurred on the night of 15 July).

Erosion:
Many lakeside reserves and public footpaths suffered damage from erosion
(Figure 1), undermining a section of sewer pipes and a carpark on the northern shore.
The Kuratau Boat Ramp also lost some of it's foundations.

Stormwater:
Number of outfalls damaged, and capacity of system reduced (fortunate
that there was no heavy rain event during this time).

Sewerage:
Five Mile Bay and Waitahanui worst affected. Infiltration into system was
four times the normal flow. Septic tanks still exist in the area, many overflowed into
main system due to the high lake levels (not designed for those levels). Not too many
problems experienced with this increase.

many jetties/boat ramps became unusable.

Figure 2: Shoreline erosion at Lake Taupo (due to high lake levels)

Note: maximum lake levels for planned storage are 357.14 m and 357.25 m, for
summer (January-March) and winter (April-December) respectively. The maximum
level is lower in summer because cyclonic rainfall is more likely then.

Karapiro Flows

Environment Waikato and ECNZ worked around the clock to ensure that the Karapiro
Hydro Dam was used to the best advantage possible (i.e. to minimise flooding
downstream). The cascade effect and uncontrolled tributary inflows through the already
full dams, made this process difficult. It became increasingly apparent that to
effectively manage flood flows through the lower Waikato system, the flood managers
needed a constant flow from Karapiro. Once the Taupo Control Gates were opened to
maximum, a steady flow through the dams was finally achieved.

It has become apparent that the tributary inflows into the Waikato system
between Karapiro and Mercer were significant (0.4 cumecs / km2 between
Karapiro and Hamilton, as opposed to 0.12 cumecs / km2 between the
dams).

It is apparent that the volume of water entering the Waikato system from
minor tributaries between Karapiro and Mercer played a relatively major role
in contributing to the flood levels experienced on the lower Waikato.

The peak discharge from Karapiro was 550 cumecs on 12/7/98. At Hamilton, the flow
had increased to 750 cumecs due to another 200 cumecs entering the system between
the two locations.

Without the effect of the hydro dams and the option to use Lake Taupo
as a flood storage area, peak flood levels at Hamilton, Huntly, and
Mercer would have been 1 metre, 0.5 metres, and 0.1 metres higher
respectively.

Lake Waikare/Whangamarino Wetland Gate Settings

Both the Lake Waikare and Whangamarino Wetland Control Gates were closed on 10
July when the Waikato River level exceeded the water level of the wetland (both gates are either closed or opened simultaneously). Between 11-20 July, Lake Waikare steadily
increased from 5.60 metres to peak at 6.29 metres (highest level since 1958) due mainly
to the high inflows from the Matahuru Stream which continued at higher than normal
levels for most of this period (due to further rainfall).

Many farms that fringe the lake were inevitably flooded as the local stopbanks (i.e.
private) were overtopped. Likewise, some farms fringing the Whangamarino wetland
were also inundated due to the swollen Maramarua and Whangamarino Rivers which
continued to infill the wetland above private stopbank levels while the control gate remained
closed.

Since the establishment of the Lake Waikare gate, the lake level can now be artificially
controlled between 5.50-5.65 m. In pre-gate days, the average level of the lake hovered
around 6.5 m (0.21 m higher than the flood peak in July 1998). The highest lake level
on record is 8.38 m (1958). The design flood level of the scheme (when lake
floodwaters actually flow over farmland and into the wetland) is 7.37 m. In this event,
approximately 1.7 km2 of land was flooded by Lake Waikare.

The Whangamarino Wetland area swelled from its normal 17 km2 to 67 km2 in this
event as opposed to 126 km2 in the 1958 flood. Without the presence of the gates, the wetland level would have been equal to the Waikato River level of 6.11 m or an extra
73 km2 of land underwater, potentially totaling 140 km2).

Flood Warnings and Alerts

The flood alert levels in the Taupo, Waipa, and Waikato River systems were quickly
raised from 3 to 5 from the evening of 9 July when it became obvious that extremely
high river flows were being generated in the upper catchments, and heavy rain was
continuing to fall.

The flood alert levels for the Hauraki/Coromandel Rivers reached 4 in some areas.

The Flood Operations Centre was activated on Thursday 9 July at 11:00 pm. From that
point on (until Monday 20 July), river levels and rainfall patterns were monitored by staff 24
hours per day. The telemetry system switched from normal day/night mode into alert
mode meaning that sites were being polled (updated) every hour, instead of the usual 3
hours.

Environment Waikato flood duty staff kept in constant contact with district councils, ECNZ and residents living on or near the Waikato/Waipa Rivers, advising them of expected peak
levels and times. Predicting the arrival of the peaks became a difficult exercise with the
onset of more heavy rain, and the already high river levels from the previous event.

Regular contact was also made with the Paeroa Flood Response Officers in regard to
the Piako River flood situation.

Flood warning recipients were notified by phone, fax, email, and/or paging. The 0832
Infolines were also operational during the event, and staff encouraged customers (where
relevant) to use it as much as possible. Telecom has since advised that the
Waipa/Waikato Infoline had a 335% increase in the number of calls between 1-20 July,
taking pressure off key staff in the flood control centre.

Media Coverage

Media coverage of the event was extensive, shifting from a flood warning perspective in
the initial stages, through to the effects of the flood on farms as far as recovery and
compensation claims were concerned towards the end. In general, the link between the
flood duty staff and the media worked extremely well. While the main media were radio
and newspapers, some staff were interviewed by both TV 1 and TV 3 reporters. A
compilation of TV news clippings relating to the flood event has been produced for future reference.

Flood Management Procedures

This flood event was managed with the assistance of the following documents:

Due to the complexities of this event, decisions at times were made with limited
information available. Circumstances also changed very quickly, and were
unpredictable.

The guidelines outlined in the above reports have been tailored specifically for the
whole Waikato River system (i.e. including Lake Taupo, Waipa River, and other
tributaries and designated flood storage areas). These are derived from many years of
historical data, knowledge, and experience by the various scheme managers.

Flood Effects:

Taupo/Waikato/Waipa Rivers
The peak flow at Tauranga-Taupo (estimated to have a probability of occurrence in any
one year of 5% (20 year flow) inundated SH 1 and the road was closed for some days.

The peak flow in the Awakino River Taupo (estimated to have a probability of
occurrence in any one year of 20% (5 year flow) inundated SH 3, and the road was
closed due to flooding and slips.

The peak flows in the upper Waipa were the highest recorded since 1958. The peak
flows at Otewa and TeKuiti were estimated to have a probability of occurrence in any
one year of 1% (100 year flow), and a peak flow at Otorohanga of about 2% (50 year
flow), and at Whatawhata approximately a 1.5% probability of occurrence (80 year
flow).

The peak flows in the lower Waikato system from Ngaruawahia to Mercer hovered
between a 2% (50 year flow) and a 1% (100 year flow) probability of occurrence,
which were in places higher than the 1958 peak flow. The peak flow at Mercer reached
a level of 6.11 metres, inundating unprotected areas of the floodplain in this locality,
including the Morrison Road basin and the Mercer riverbanks which are located outside
the scheme area. The Rangiriri Spillway operated as designed for the first time in 28
years (Figure 3).

Figure 3 : Rangiriri Spillway allowing water to flow over State Highway 1 as per design. The road remained open throughout the event.

The nature of the second deluge was to produce high flows in all catchment areas (i.e.
upper and lower) due to record rainfall that was fairly widespread. The flood wave
increased in severity as it moved downstream due to the heavy input from the middle
and lower Waikato tributaries. This was paramount in the lower Waikato as the peak
flow from the Waikato Hydro system passed through Ngaruawahia before the Waipa
peak flow arrived.

Over time, controlling the timing of the peaks proved extremely difficult due to
continued rainfall, which either re-elevated river levels or attenuated the peak flow.

This meant that the Waikato peak flows through Hamilton, and the Waipa peak flows
through Whatawhata were sustained at a very high level for some time.

The normal time of travel for flows between Otorohanga and Whatawhata is about 16
hours, but this peak took about 90 hours to reach Whatawhata.

Hauraki/Coromandel Rivers
The peak flow in the upper Waitoa river system equated to a 5% (20 year flow)
probability of occurrence in any one year.

The Piako Ponding Zones were used for the first time as per the Piako River Scheme
design, with significant flooding occurring in these zones.

Provisional Flood Damage - Rural
It is estimated that 1000-1500 hectares of farmland were flooded, predominantly in the
lower Waikato area. Information provided from Federated Farmers indicate that about 9
farms were completely under water, with another 30-40 partially affected.

Provisional Flood Damage - Residential
The following residential areas were reported to have suffered some form of flood
damage:

It has been estimated that the costs for the Waikato Region in regard to state highway
damage will lie somewhere between $7-9 million. This includes costs of about $5
million for the large slip on SH 3 near Mohoenui. The total cost is expected to top $17 million, including damage to the protection scheme, infrastructure, farms, and property.

Review Programme (Where to from here)

Full Review

Flood Event
Flood Management
Scheme Management

Debriefings

Internal
Key Agencies
Community

Reporting

Auditing

Scheme Repairs

Longer Term (Improving our Performance)

Addressing identified issues

Review of the the Flood Rules and Flood Warning Manual

Scheme Protection Standards

Hazard Management

land-use planning
mapping of floodplain areas

Establishing Emergency Management Groups

Summary and Conclusions

The main causal mechanisms for this flood event were:

Above normal rainfall - the highest on record for July. Note: June was also above normal in most areas

Saturated ground conditions affecting the whole river catchment

Back-to-back storms which meant river levels could not return to normal

Flood flows in the Tauranga-Taupo, Tongariro, Waipa, and Waikato River systems in
this event have been regarded (at some sites) as being larger than the 1958 flood, but
not quite rivalling the 1907 event. Hence, the Alert Level assigned to these systems
reached 5 (major flooding, minor roads flooded, and major flows in river system).

Flood warnings and management systems generally worked very well, and the Lower
Waikato Waipa Control Scheme operated better than design in many places. The
general reaction from the community, both from the urban and rural sector, has been
extremely positive.

It should be noted that the majority of the previous major flood events that are commonly
referred to, occurred during the summer period (i.e. cyclone originated), whereas this
event has been out of the norm occurring during the winter months.

MetService have since indicated that a very warm north Tasman Sea provided the
energy for the successive depressions that formed during July, bringing with them the
active rainfall bands that resulted in this flood.